I've casually free dove all my life (not to any significant depth), and I was shocked to hear that blackouts can happen even at shallow depths. I definitely have a lot more respect for this sport and will avoid doing it alone in the future.
Shallow water blackout is a phenomenon that occurs when lungs expand while ascending from a deep dive. The contrast from deep to shallow is what's dangerous. Oxygen is rapidly pulled out of your bloodstream back into your lungs as they expand. It happens quickly (and unexpectedly if you're not already aware of it). Of course you can stay too long at a shallow depth, but that's a simple, predictable linear exhaustion of oxygen.
